1. Field of the Invention
The present invention relates to an apparatus for direct or indirect application of a liquid or pasty medium onto a traveling material web, notably of paper or cardboard, including at least one long doctor element for application and/or metering of the medium.
2. Description of the Related Art
Particularly in the paper industry, the tendency is noted to produce increasingly wider material webs. Their width frequently reaches several meters, for instance 6 to 8 meters, and occasionally up to 10 meters or more. A corresponding length is then required of the doctor bars or doctor slats used in a coating or sizing station of a paper or cardboard manufacturing system for applying a sizing solution or coating color onto the paper or cardboard webs and to meter the applied substance. A coating, for example of white or colored pigments, is applied onto the paper or cardboard webs in such a coating station so as to close pores that are still present in the material and to cover and level out the surface of the material. Obtained thereby can be continuous and smooth surfaces on the paper or cardboard, such as needed for a demanding and sharp image reproduction, for example, in copiers or in printing plants.
Doctor elements are used in the direct and indirect application of coating substance. In indirect application, the substance is first applied onto an applicator roll, from which it transfers onto the material web. In direct application, the substance is applied directly onto the material web. The doctor elements serve in both cases to even out the applied substance, to safeguard its exact metering, and to strip, i.e., doctor down, the substance surpluses. Doctor bars are usually mounted rotatably in a doctor bed and rotated by use of a rotary drive arrangement. Also employed for these purposes, however, are stationary doctor elements, for example, doctor slats or doctor blades.
Doctor elements are generally made of steel. They frequently have a hard chrome plating on their surface or feature an outside layer of ceramic or fiber material, for example, glass fiber, carbon fiber, or fiber-reinforced material. Their surface may be smooth or rough or, for volumetric application, provided with a surface profiling, e.g., a grooving pattern. Also known are wire-wound round-section rods, so-called wire doctors.
Doctor elements are customarily fabricated in a single piece across their entire length. Relevant patent documents are German Patent Document No. DE 195 15 754 C1 and European Patent Document Nos. EP 0 453 427 A2, EP 0 454 643 B1 and EP 0 674 047 A2. Stringent requirements are likewise imposed on the manufacturing accuracy of the doctor elements. Difficulties particularly arise with large lengths of the doctor elements, such as lengths of several meters as needed for modern paper and cardboard manufacturing systems. To begin with, the doctor elements are very slender, frequently having diameters, in the case of doctor bars, of as little as 9 to 20 millimeters, but maximally 80 to 200 millimeters. The doctor elements must be handled very carefully both in their manufacture and in shipping or installation in order to avoid damage to the doctor surface, bending and other damage. Even the slightest bending, such as might occur, e.g., in the shipping of very long, single-piece doctor elements, is not acceptable in view of the high accuracy requirements. Moreover, the machines used in the manufacture of the doctor elements must be adapted to the large length of the doctor elements. For example, sagging of the bars, slats or tubes serving as initial material, such as may occur very easily with the large lengths concerned, must be prevented from resulting in finishing inaccuracies. In any case, it is difficult to provide bars or tubes of the required lengths which possess sufficient straightness. Lastly, the different customer needs make it necessary to manufacture the doctor elements individually in the desired length or design.